Simple cooling tower model

Most thermo hydraulic processes employ some means of heat rejection. This can be done by means of a number of processes for which evaporative cooling is a frequently used means. Although the heat rejection process may not necessarily be the focus of the study, the behaviour and interaction of the heat rejection plant with the main process plant can influence overall system performance. It is therefore necessary to quantify the performance of the heat rejection plant by some means.

A simple Flownex® evaporative cooling tower model was developed to address the need to quantify the performance of evaporative cooling processes. Evaporative cooling is largely dependent on the ambient air’s wetbulb temperature as well as the ability of the evaporative cooling device to bring water into contact with the cooling air to allow evaporation to take place. In order to determine the evaporation and ultimately the heat transfer of the cooling tower, calculation of the changes in the air’s psychrometry is required. Flownex’s® ability to perform psychrometric calculations gives the software the ability to do just that.

Theory based on the factor of merit model [1] was employed to characterise the cooling towers’ ability to bring water into contact with the cooling air to allow evaporation and heat transfer to take place. This is done by means of specifying a factor of merit which is an indication of the cooling towers effectiveness. This theory was coded in a Flownex® script and along with other basic components used to set up a simple Flownex® cooling tower model.

The basic cooling tower model consists of an air and water side as well as some script components. The air side fluid consists of a two phase water and air mixture to allow psychrometric calculations. The air inlet conditions are specified by means of specifying the atmospheric pressure, drybulb temperature as well as air and water mass fractions to obtain the required inlet wetbulb temperature. A fan or fixed mass flow can be connected to the air side to obtain/specify the air mass flow rate. The water side uses two phase water as the fluid. On the water side the water inlet temperature and flow rate is specified or obtained by means of connecting the cooling tower to a network.

In order for the user to characterise the cooling tower model, the factor of merit (F ranging from 0-1) in the script needs to be adjusted until the required water outlet temperature (measured or obtained from manufacturers data) is obtained for the given water and air inlet conditions and flow rates.

The cooling tower heat transfer as well as mass flow water evaporated during the cooling process can be obtained from the script while the air and water outlet conditions can be obtained from the outlet elements on the air and water side respectively.

Note: In the attached example network the cooling tower water side was separated from the rest of the simulated network (process plant) by means of data transfer links. This was done for the following reasons:

Single phase water can be used in the process plant network, thus increasing solver speed and simplifying the fluid.

An open container could be connected to the bottom of the cooling tower, simulating the cooling tower pond.

Disconnecting the cooling tower model from the process model by means of data transfer links in some cases stabilises the cooling tower model and assists in solver convergence.

References:

[1] Bluhm. S, J. and Whillier. A. The design of spray chambers for bulk cooling of the air in mines. Journal of the South African Institute of Mining and Metallurgy. August 1978.